The phrase "ascertain the number" is correct and usable in written English.
You can use it when you need to determine or find out a specific quantity or figure.
Example: "Before we proceed with the project, we need to ascertain the number of participants who will be attending the event."
Alternatives: "determine the number" or "find out the number".
"We cannot ascertain the number of people killed for now, and moreover we cannot say what or who was responsible for the attack," Mr. Adenaike said.
Managers can also ascertain the number, times, and nature of a worker's telephone calls, monitor e-mail, or track the number and nature of Web sites an employee accesses.
If the links between Beijing and Pol Pot are censored at the Khmer Rouge trials; if there is no serious effort to ascertain the number of victims of the Cultural Revolution in China; if the archives of the Gulag or the war in Chechnya have still to be protected and sometimes concealed by the Russian authorities, what can we expect the attitude of these countries to be toward massacres to come?
He raised objections to Project Tiger's census method of counting pugmarks, to ascertain the number of tigers back in 1986.
It is difficult to fully ascertain the number of teenagers in the United Kingdom who have more than one pregnancy before the age of 20 years.
A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy.
We use this to proxy desired family size which is asked in demographic surveys to ascertain the number of children that people want rather than the number they actually have.
We performed a series of mating experiments to ascertain the number of oocytes ovulated and the potential for embryo development in vitro to the blastocyst stage and in vivo to Day 12 of gestation.
